The present invention is directed to a jaw for use in a sealless strapping machine for forming a sealless seal. More particularly, the present invention is directed to a one-piece jaw for forming a sealless seal.
Strapping machines are in widespread use. Typically, strapping machines are designed for use with metal (steel) or plastic strapping. In use, strapping is wrapped around the load, tensioned and sealed to itself. In use of metal strapping, the seal can be formed by use of a separate seal that is positioned around the strap and crimped onto the overlapping courses of strap. The seal can also be formed by gripping the strap material sections between a punch and the strapping machine jaw and cutting portions of the over- and under-lying courses of strap material into one another to interlock the strap courses. Such a seal is referred to as a sealless seal.
This action creates a locking action that provides additional strength to the strap, so that the tension in the band is maintained. The geometry of the jaw is highly engineered and does not lend itself to change. The jaws, by the nature of the design, experiences extremely high fluctuating stresses, that can result in fatigue failure if the maximum tensile stresses at the shear section exceeds half the shear yield strength. There are tremendous gains to be had if the stresses stay at or below ½ the yield strength.
Failure of the jaws can occur by fracture at the interface of the punch with the jaw structure. Again, this is due to the high localized stresses that are exhibited at this juncture. In order to address this, it has been thought that a build-up of material (e.g., making the jaw thicker) at the juncture would preclude or lessen the chances of the jaw failing. While this has increased the jaw life, it results in other undesirable characteristics, namely, the jaws have become too large and heavy and the space for the strap to move into the jaw area and for the strap to reside during the sealing operation is too small.
Another approach has been to use a two piece design that relies on improving wear performance through the use of two different materials (one for the bearing surface, e.g., the punches, and one for the bulk). Such as design lowers the shear cross sectional area that is subjected to the highest loads, and hence lowers fatigue life.
Known punches, e.g., bearing surfaces, are such as that shown in U.S. Pat. No. 6,554,030 to Cheung, et al. Each the fixed and movable punches include a plurality of punch heads. The punches are mounted to the jaw by fasteners to permit removing the punches for replacement, repair or maintenance. One drawback to this design is that the bearing surfaces (punches) are mounted to the jaw structure using fasteners, such as bolts. The bores or holes that are formed to secure the punches to the jaw structure also create areas that can be susceptible to failure.
Accordingly, there is a need for a jaw seal arrangement in which the jaw will not fail under load and that provides sufficient space in and around the punches for the positioning of strap in the jaw for sealing.